1. Field of the Invention
This invention relates to semiconductor devices having structures which move and change the electrical properties of the devices, processes for making such devices, and methods for using such devices in sensors.
2. Description of Related Art
Semiconductor devices having movable or deformable structures which change the electrical properties of the devices have been proposed but have technological problems which limit their development. One problem with such devices is that their fabrication requires non-standard processes which decrease yield, increase expense, and make integration with standard MOS devices difficult. Additionally, the operation of such devices has been misunderstood. For example, a field effect transistor (FET) with a movable gate has a threshold voltage which depends on gate position. This fact is central to making practical movable gate transistors but has not been generally recognized.
U.S. Pat. No. 5,155,061 issued Oct. 13, 1992 (O'Connor et al.) discloses a semiconductor pressure sensor incorporating an ungated metal-oxide semiconductor FET. The pressure sensor uses a deep (about 10 .mu.m) depression in a silicon substrate to form a gap between a suspended gate and an underlying channel in the substrate. The deep depression is made before a source, drain, and channel are formed and interferes with standard MOS processes such as spinning on photoresist and photolithography which work best on nearly planar surfaces. Using standard MOS processes, a 10 .mu.m depression would cause a significant reduction in the manufacturing yield of operable devices.
Further, an FET with a 10 .mu.m gap between the gate and the channel would have a threshold voltage on the order of hundreds of volts. Such gate voltages are too high for most applications. O'Connor et al. fails to teach how to reduce the gap to provide a more practical device. O'Connor et al. also fails to address the effect of electrostatic forces on the gate. Specifically, a bias voltage between the gate membrane and the substrate attracts the gate membrane toward the substrate. This attraction is not very significant with a 10 .mu.m gap, but for more practical FETs having sub-micron gate-channel gaps electrostatic attraction is important.
U.S. Pat. No. 4,812,888 issued Mar. 14, 1989 (Blackburn) describes a FET having a smaller gap. Blackburn forms a movable gate for the FET by depositing a gate material such as polysilicon or metal over a sacrificial layer and then chemically etching the sacrificial layer from under the gate diaphragm. Blackburn's FET is more practical than O'Connor's since Blackburn's FET's has a smaller gap and a threshold voltage that can be within a reasonable range. However, Blackburn's fabrication process is defect prone. Chemical etching of sacrificial layers tends to leave a residue on and under the gate diaphragms which cause threshold voltage errors, and the long etching time required to remove material from under a diaphragm can damage other regions of the device. Also, many applications of moving gate field effect transistors require a vacuum between the gate and underlying substrate. Blackburn's fabrication process requires forming a vacuum in a cavity and then sealing the cavity, but no commercially repeatable vacuum sealing technique is currently available.
Blackburn also demonstrates the prior arts' incomplete understanding of MOGFET physics. In particular, Blackburn fails to disclose the strong sensitivity of threshold voltage to gate deflection and does not address the issue of significant electrostatic forces on the gate diaphragm. Known deposited gate diaphragm materials as used in Blackburn are not single crystal and have less than ideal mechanical properties. In particular, these materials are not thermally matched with the substrate, have mechanical hysteresis, and tend to permanently deform. Additionally, the active semiconductor regions shown in Blackburn are not ideal for providing a signal indicating the deflection of the gate diaphragm. Failure to account for these issues leads to manufacturing, reliability, and performance problems.